Poison, Shadow and Farewell (Your Face Tomorrow, part 3) By Javier Marías trans Margaret Jull Costa

A perfect Spanish spy

Jack Deza, the narrator of Javier Marías's 1500-page trilogy, Your Face Tomorrow, is an acute observer who reflects on other people's behaviour, rather as a novelist hopes to do.

He works for a secret department of MI6: after interviewing people or watching videotapes, he assesses their character and their future behaviour. In this final volume, Deza the intellectual becomes a man of action and discovers his own "face tomorrow". And the cultivated, sophisticated European, ever the voyeur analysing others, finds that, when he acts, he engages in foul violence. "Poisoned" by his boss Tupra, he enters into "shadow" before bidding his readers an ambivalent "farewell".

Marías's first big success 20 years ago, All Souls, mocked the follies of Oxford academics, but failure and deceit were the dark accompanists of the laughter. The 1990s saw three other novels greeted with major international awards, notably the 1997 Impac Prize for A Heart So White (a novel with the most desolate opening I know).

Marías is something of a one-off in Spanish letters. His philosophising paragraphs, the fierce violence interrupting reflection, his comic set-pieces and apparent lack of plot reflect his international influences, from Tristram Shandy (which he translated) to Shakespeare (source of his titles) or film noir. Marías writes modern tragedy, yet his tone is often high comedy.

In the trilogy, he recovers the first-person narrator of All Souls, now recruited in classic style by an Oxford don to work for the secret service. The mood has darkened: Deza has married, had two children and separated. No longer a university lecturer, now he witnesses, even engages in, blackmail and intimidation. Deza now sees in his spy job how the dirty tricks that were justifiable in the war against Hitler continue in peacetime. It is partly John Le Carré territory, where the secret state beyond democratic control acts ruthlessly to protect the rich. No longer a visitor in genteel Oxford with a home in Madrid, Deza is now an exile who has lost his bearings in a foreign land.

The title of this ambitious trilogy of ideas comes from Prince Hal in Henry IV and refers to the change in the Prince when he becomes king and thrusts Falstaff aside. Though the king's betrayal of his bosom friend is for reasons of state, it is personal too. Betrayal, in both the public and private spheres, is the central theme of the trilogy.

Marías's method is to take ideas then examine them from multiple points of view. In the first volume, he documents the heroic refusal of revolutionary leader Andrés Nin to betray his colleagues under torture; in the third volume, he weighs this against the narrator's father's colleague, who betrayed him in the same Spanish Civil War. In between, other stories of betrayal cast light and nuance on the theme.

In the second volume, in the central scene of the trilogy, Tupra memorably assaults and terrifies the gross Spanish diplomat De la Garza in a nightclub toilet. Tupra's justification is that his brutality has averted much greater violence in the future: the classic argument that the future end justifies dirty means today. Then, in this volume, Deza has to decide, in relation to his wife's lover, the also gross Custardoy, whether to imitate Tupra's behaviour or follow the ethical example of his father.

Marías is not quite the novelist he appears to be. He is so playful and virtuosic that he at first seems a brilliant postmodern conjuror, throwing all points of view into the hat, but not himself taking an ethical position. In fact, he is very serious about "seeing things as they really are" (a recurrent phrase) and establishing what happened in history. He shows how knowledge is complicated, but does not believe the past is unknowable. The trilogy poses how the question of the experience of those extreme times of civil and world war is still relevant to our decisions today.

Among a relatively small cast of fascinating characters, all with complex identities, Marías's finest creation is Bertram Tupra. Enormously attractive to both Deza and readers, Tupra is an amoral cynic. A perfect agent of political power, cultured and urbane by preference, he is prepared to use ruthless force to defend his masters' interests. Tupra is James Bond, but filled out: not just charming and glamorous, but also an unscrupulous thug.

Marías's style is slow, hypnotic and baroque. His trilogy shares with symphonic music the development of motifs, which it plays repeatedly with variations. I am not the only admirer of Marías to have hurled down a book of his in frustration at his long philosophical tangents, then rushed to pick it up, compelled by the knowledge that his winding ruminations lead into stunning pieces of action. In general, the digressions are fascinating stories in themselves, though some – and here the book-throwing temptation lurks – are infuriatingly long-winded.

In highlighting the dominant theme of betrayal, I do not wish to diminish the trilogy's richness of themes. The nature of political power, fear, the use of violence or the danger of words ("Careless talk costs lives") are just some of the other questions that Marías tackles.

The trilogy is full of jokes, anecdotes, literary references, real people passing through (Jayne Mansfield or Ian Fleming – curiously, more ghostly than the fictional characters), even photos and posters. It is translated with fluent skill by Margaret Jull Costa. Marías, you feel, enjoys his writing and that helps readers to revel in an outstanding book that rounds off one of the most thoughtful and inspiring fictional works of the last decade.

Michael Eaude's books include 'Catalonia: a cultural history' (Signal)